1. Field of the Invention
The present invention relates to semiconductor fabrication and, more particularly, to compensation for proximity effects that result during fabricating integrated circuits.
2. Description of the Related Art
Photolithography is an important part of fabricating integrated circuits. In photolithography, a light opaque pattern imprinted on a mask or reticle interposed between a radiation source and a photosensitive resist (photoresist) layer on a semiconductor wafer. If the photoresist polarity is positive, the exposed portions of the photoresist with respect to the radiation source are easily dissolved or otherwise removed in a subsequent development step. The unexposed portions of the positive photoresist remain polymerized and will not be removed during the development step. After the exposed portions of the photoresist are dissolved and removed, the resulting wafer uses the remaining patterned photoresist layer as a protective layer to, for example, block the deposition of dopants or to prevent etching of one or more layers underlying the remaining photoresist.
One type of projection photolithographic process uses a mask (containing the entire wafer pattern) which is spaced close to the wafer. In this process, no lens system is required to focus the mask image onto the wafer surface. Another type of projection photolithographic process uses a mask spaced away from the wafer, wherein a lens system interposed between the mask and wafer is used to focus the pattern of the mask onto the entire wafer.
An improved type of projection photolithographic process uses a reticle, which contains a pattern for a single die or a relatively small portion of the wafer. This process uses a stepper, wherein the reticle is mounted typically 50 centimeters to 1 meter from the wafer, and a lens system focuses the reticle pattern on a small portion of the wafer to expose the photoresist. The wafer is then slightly shifted relative to the reticle image, and the exposure process is repeated until substantially the entire wafer has been exposed by the same reticle in a repeated pattern.
As is well known, a major limiting factor in image resolution when using any photolithographic process is the diffraction of light, where light bends around the mask or reticle pattern. Due to diffraction, the mask or reticle pattern is slightly distorted when the pattern image is projected onto the wafer surface. This distortion is often referred to as the proximity effect.
With conventional projection photolithographic methods using conventional masks and reticles, line widths of lines within a dense pattern of lines formed on a wafer surface are narrower than line widths of isolated lines, even though all line widths on the mask or reticle are identical. Such is the case where a positive photoresist is used and the opaque portions of the mask or reticle correspond to the lines and other features to be formed on the wafer surface. Where a negative photoresist is used, causing the clear portions of the mask or reticle to correspond to the lines and other features formed on the wafer surface, the effect would be the opposite.
Thus, the resulting wafer contains feature sizes that are dependent upon whether a feature is isolated or within a dense pattern. This results in unpredictable feature sizes. One skilled in the art of integrated circuit design will be aware of the various problems which may result from unpredictable feature sizes, such as differing electrical characteristics. In any case, the change in feature size due to the impact of other nearby features is known as the proximity effect. Although the following discussion assumes the features are lines, the features can have any geometric shape and are not limited to lines.
FIGS. 1A and 1B are diagrams that illustrate the proximity effect with respect to a simple metalization process. In FIG. 1A, a wafer 10 has an unpatterned layer of silicon dioxide 12 formed on its surface. A metal layer 14, typically aluminum, is then deposited on the surface of wafer 10 over the silicon dioxide 12 using conventional techniques. A layer of positive photoresist is then spun onto the surface of the wafer 10 to completely coat the surface of the wafer. Using well-known techniques, the wafer surface is then selectively exposed to radiation through a reticle in accordance with a reticle pattern. The reticle pattern is represented by light blocking portions 16 and 18, which block light (radiation) from a lamp that is used to expose the photoresist. Downward arrows represent partially coherent radiation 20 from the lamp. A lens 22 focuses the image of the reticle onto the surface of wafer 10. The x-axis of the graph in FIG. 1A represents the distance along the wafer 10 surface, and the y-axis of the graph represents the resulting light intensity on the wafer 10 surface.
As seen by the intensity of light impinging upon the wafer surface, a certain low level of light intensity exists under light blocking portions 16 and 18 due to the diffraction of light, whereby the light waves traveling in straight paths bend around light blocking portions 16 and 18. Thus, additional area of the photoresist is exposed to light due to the diffraction of light. As a result of the diffraction, the light waves from radiation 20 have constructively and destructively interfered with one another as a result of the diffraction of light. Hence, where the light intensity is increased due to constructive interference, the photoresist will be even more exposed. The extent of diffraction is a function of light coherency, numerical aperture of the lens used, and other factors, as known in the art.
It is assumed for purposes of illustration that any photoresist exposed to light above a threshold intensity level L.sub.TH will be dissolved away during development of the photoresist. This threshold light intensity level L.sub.TH is represented on the y-axis of the graph in FIG. 1A.
After the wafer 10 is sufficiently exposed to the light pattern and after the exposed photoresist has been removed, the photoresist portions 24 remain. In this example, the width of photoresist portions 24 is 0.74 microns. Next, the exposed metal layer 14 is anisotropically etched, using well-known techniques, and photoresist portions 24 are thereafter removed with a photoresist stripper. The remaining oxide 12 may then be removed as desired. What remains is a metal pattern including parallel metal lines 26 and 28, whose geometrics are dictated by the geometries of light blocking portions 16 and 18 and by the spaces between the light blocking portions. The width of light blocking portions 16 and 18 corresponds to metal line widths of 0.74 microns. In this example, the pitch or distance between the centers of metal lines 26 and 28 is three microns. Hence, in this example, the pitch of three microns for parallel metal lines 26 and 28 is large enough so that the diffraction from light blocking portion 16 does not influence the shape of metal line 28 and the diffraction from light blocking 18 does not influence the shape of metal line 26.
FIG. 1B illustrates a similar example where metal lines 30, 31 and 32 being patterned have a smaller pitch (i.e., greater density) than did the metal lines 26 and 28 of FIG. 1A. Here, the pitch between metal lines 30, 31 and 32 is 1.5 microns. As illustrated, the resulting lengths of metal lines 30, 31 and 32 are less than 0.74 microns, even though the widths of light blocking portions 40-42 of the reticle are identical to the widths of light blocking portions 16 and 18 in FIG. 1A. This is because light blocking portions 40-42 are situated sufficiently close to one another such that the diffraction from light blocking portions 40 and 42 cause a greater amount of photoresist under the center light blocking portion 41 to be exposed above the threshold intensity L.sub.TH. Also, the diffraction effects from light blocking portion 41 cause a greater amount of photoresist under light blocking portions 40 and 42 to be exposed above the threshold intensity L.sub.TH. Consequently, the metal line 31 is narrower than metal lines 30 and 32, since the length of the metal line 31 is reduced on both sides by the diffraction due to the light blocking portions 40 and 42.
The undesirable consequences of the proximity effect discussed above with reference to FIGS. 1A and 1B are for a representative example. However, it should be understood that the proximity effect is inherent in photolithographic processing regardless of feature geometries or materials.
Additionally, etching features on a semiconductor wafer also leads to undesirable proximity effects. Here, the proximity effect is due not to diffraction of light, but instead due residue polymer or transport of removed material from the surface of the semiconductor surface, for example. In any case, the proximity effect causes the etching process to yield different etch rates depending on whether a feature being etched is isolated from or within a dense pattern of other features. For example, when the features are formed in a dense pattern, the resulting etch rate is higher, whereas when the features are somewhat isolated from one another, the resulting etch rate is lower.
Accordingly, when lithographic and etch processing are utilized in fabricating integrated circuits, the impact of the proximity effect should be monitored and corrected. The monitoring of the proximity effect is conventionally by measuring the lengths of the resulting features that are formed by the lithographic and etch processing. Conventionally, the lengths of the resulting features from the lithography and etch processing are measured by a resistance approach or an electron microscope. The electron microscope can be used to accurately measure lengths but is expensive, time consuming and destructive to the integrated circuit, and thus is often not a practical solution. With the resistance approach, the sheet resistance of the formed features is measured. The sheet resistances are then used to compute the lengths of the formed features. Thereafter, these measured lengths are compared to the intended lengths for the features, and when the deviation is beyond a tolerance level, Optical Proximity Correction (OPC) is performed to alter the mask or reticle to compensate for the proximity effect.
One problem with the conventional resistive approach for measuring the lengths of the features is that in many cases the resistance measured does not accurately correspond to the length of the feature being measured. When this occurs, the lengths that are computed from the measured resistances have a loss of accuracy. As a result, the proximity effect is not able to be accurately quantified.
Thus, there is a need for improved techniques for measuring lengths of features formed on a wafer so that proximity effects can be accurately quantified.